1. Field of Invention
The invention relates to a thin-film gas diffusion electrode (GDE) and the method for making the same. In particular, the invention relates to a thin-film GDE formed in a unitary way and the method there for making it.
2. Related Art
The gas diffusion electrode (GDE) plays an important role in chemical power sources. The GDE uses gas, such as hydrogen and oxygen, as the active material. Through the dissolving and diffusion steps, the gas active material has electrochemical reactions with the ions in the electrolyte to produce an electric current. The GDE using oxygen as the active material is called the air electrode. The combination of the oxygen GDE and the hydrogen GDE forms the main part of fuel cells.
In observation of the impacts on the environment due to the rapid population growth and industrial over-development, how to provide an environment-friendly business has become an important subject for all companies. The low-pollution fuel cells are seen as the most important energy source in the 21st century to provide a clean electric power. Such fuel cells can be widely used in electric cars, generators, and even the 3C products. The fuel cells produce electric power through electrochemical reactions but not burning. The hydrogen and oxygen react to produce electric currents, water and heat. There is almost no pollution at all.
The conventional GDE is mainly composed of three parts: a current collector, a gas diffusion layer, and an active layer. The current collector is responsible for electron transmissions. It is made of a good conductive metal. The gas diffusion layer provides a channel for gas to diffuse. It has to be kept completely dry, preventing the electrolyte from entering and blocking the diffusion channel. The active layer is the place to undergo a inhomogeneous electrochemical reactions. To accelerate the reaction speed, the active layer is often added with electrocatalysts.
The current GDE manufacturing technology is to make the above-mentioned three parts separately and combine them together afterwards. For the active layer, one first has to adsorb electrocatalysts directly onto a porous carbon substance and then add 5% to 15% of binder, which is usually a hydrophobic polytetrafluoroethylene (PTFE). They are homogeneously mixed and formed by rolling. For the gas diffusion layer, one mixes carbon powders and a high-concentration (15% to 25%) hydrophobic binder, usually hydrophobic PTFE, and performs formation by rolling. After finishing the active layer and the gas diffusion layer, they along with the current collect are combined to form a single GDE by hot-pressing. In this method, one needs to use a binder that may cause the following problems. First, during hot-pressing the binder experiences a temperature higher than its glass temperature. Therefore, it becomes mobile so that under pressure it often clogs the gaps and holes on the electrode. Therefore, larger gas channels in the gas diffusion layer are blocked, increasing the resistance of gas transportation. At the same time, the binder is likely cover the electrocatalysts disposed on the carbon powder surface on the active layer. The usage rate of the electrocatalysts and therefore the reaction rate are lowered. All the above effects significantly reduce the output power of the ensemble.
In addition, the conventional manufacturing method of the GDE inevitably requires the use of high-temperature and high-pressure processes. Each step of the temperature rising, lowering, and mold withdrawing results in limits on the production speed. The high-temperature and high-pressure processes also worry people with safety concerns.